Method for producing polyimide foam of desired density

ABSTRACT

Polyimide foam of desired density is produced by adding to a polyimide precursor comprising carboxylic and diamine components at least one foam-enhancing polar, protic additive of the formula ROH, where R is hydrogen, or C1 to C12 linear or branched alkyl or cycloalkyl, unsubstituted or substituted with halo, aryl, alkoxy or hydroxy, and heating the resultant slurry to a temperature that does not exceed about 105 DEG  C. to form a homogeneous melt. When heated to a higher temperature the melt foams and cures. The density of the foam is affected by the amount of foam-enhancing additive which is utilized.

This application is a division of application Ser. No. 07/575,982, filed Aug. 31, 1990, which is a continuation-in-part of application Ser. No. 07/466,122, filed Jan. 16, 1990, now abandoned.

This invention relates to polyimide foams and to the production of such foams in a range of densities. Polyimide foams are useful in fire-resistant thermal insulation and other applications.

BACKGROUND

Historically, there have been three general ways to produce polyimide foams. According to the process described in Lavin et al. U.S. 3,554,939, a monomer mixture composed of an ester of benzophenone tetracarboxylic acid and a polyamine, the mixture having a volatile content (defined as percent weight loss in 10 minutes at 300° C.) of at least 9%, is heated to a critical temperature at which foaming occurs contemporaneously with polymerization of the tetracarboxylic and polyamine components until the polyimide foam is formed.

In another procedure, described by Gagliani in Final Report NAS 9-14718 entitled "Fire Resistant Resilient Foams" dated February 1976, a mixture of diamines is added to an alcoholic solution of the half ester of benzophenone tetracarboxylic acid and reacted at 158°-167° F. (70°-75° C.) to form a heavy syrup. This syrup is heated in a circulating air oven at 180° F. (82.2° C.) for about 12-16 hours and then in a vacuum oven at 176°-194° F. (80°-90° C.) for 60-90 minutes, producing a polyimide precursor. Thereafter, the polyimide precursor is pulverized into a powder which is spread over aluminum foil on an aluminum plate and heated at 600° F. (315.6° C.) in an oven for 30 minutes to produce the foam. In a similar procedure reported by Gagliani et al. in Final Report NAS 9-15050 entitled "Development of Fire-Resistant, Low Smoke Generating, Thermally Stable End Items for Aircraft and Spacecraft" dated June 1977, the dried precursor powder formed in about the same manner was subjected, inter alia, to multi-stage heating, in which the powder was placed in a pressure vessel positioned within an oven preheated at 450° F. (232° C.) and held at this temperature at a reduced pressure (19.9-9.9 inches of Hg) for 15-30 minutes. The resulting foam was then postcured at 600° F. (315.6° C.) for 15-30 minutes in a circulating air oven.

The third procedure involves use of microwave radiation for converting the polyimide precursor into a cellular structure which normally is then subjected to final curing in a thermal oven. In actual practice the precursor is used in the form of a powder produced by spray drying an alcoholic solution of the tetracarboxylic and diamine components. See, for example, Gagliani et al. U.S. Pat. Nos. 4,296,208; 4,305,796; 4,439,381; and 4,599,365; Final Report NAS 9-15050 (supra); Final Report NAS 9-15484 entitled "Development of Fire-Resistant, Low Smoke Generating, Thermally Stable End Items for Commercial Aircraft and Spacecraft Using a Basic Polyimide Resin" and Final Report NAS 9-16009 entitled "Formulation and Characterization of Polyimide Resilient Foams of Various Densities for Aircraft Seating Applications". In U.S. Pat. Nos. 4,305,796 and 4,439,381 it is indicated that the polyimide precursor may vary from a `liquid resin` to a spreadable, paste-like formulation, depending upon the nature and quantity of any fillers added to the resin.

One of the deficiencies in the known methods for producing polyimide foams is that the density of the final foam product cannot be controlled readily over a very wide range. This is undesirable, because a foam having a certain chemical structure is useful in some applications only if it is very dense, but it may perform very well in other applications at a much lower density. A foam of low density should be less expensive.

It is known that the foaming of polyimides is related to the volatile by-products generated in the amidization and imidization reactions. According to U.S. Pat. No. 4,900,761 small changes in the solids content of the polyimide precursor lead to large changes in the density of the resultant foam, but the exemplified changes are random and unpredictable. It is also known in the art to incorporate blowing agents of various types into the polyimide precursor, in order to decrease the density of the foam. For example, U.S. Pat. No., 4,476,254; 4,518,717 and 4,621,015 disclose the incorporation of various solid hydrates, such as oxalic acid dihydrate. The incorporation of other solid blowing agents, such as azocarbonamide and boric acid, is disclosed in U.S. Pat. No. 4,506,038. The incorporation of glass microballoons, which may increase the foam density, is described in U.S. Pat. No. 4,353,998. The incorporation of such blowing agents, however, can lead to an irregular cellular structure in the foam because of the inhomogeneity of the mixture, and residual blowing agent can adversely affect the physical properties of the resultant foam.

SUMMARY OF THE INVENTION

Accordingly, it is one object of this invention to provide a method whereby the density of the foam produced from a polyimide precursor can be predictably and reproducibly controlled over a wide range. It is another objective to provide polyimide foams in a range of densities which contain no solid blowing agents or residues thereof.

According to the present invention, polyimide foams of superior quality and in a range of densities can be produced by incorporating a polar protic foam-enhancing material as an additive to a foamable polyimide precursor composition.

In accordance with one aspect of the invention, there are provided polyimide foams in a range of densities which display improved flexibility and homogeneous cell structure. According to another aspect, foam in a range of densities is produced by a process which comprises making a solution or slurry by adding to a polyimide precursor at least one polar protic foam-enhancing additive of the formula ROH, where R is hydrogen, or a C₁ to C₁₂ linear or branched alkyl or cycloalkyl radical, which may be unsubstituted or substituted with halo, aryl, alkoxy and hydroxy. The polar protic foam enhancing additive need not be miscible with or act as a solvent for any of the components of the precursor composition under ambient, i.e. room temperature, conditions. However, when the additive is heated with the polyimide precursor to a temperature below that of foam production, the mixture first begins to soften, dissolve and ultimately forms a homogeneous, transparent solution, opaque suspension, or "melt." At this stage, the temperature of the melt ordinarily should not exceed about 105° C. Preferred melt-forming temperatures are in the range of about 50° to about 100° C., most preferably about 60 to about 80° C. The melt-forming temperature will vary somewhat of course, depending upon the nature of the polyimide precursor components.

DETAILED DESCRIPTION

The polar protic foam-enhancing additives ROH include water, i.e., where R is hydrogen, and also alcohols, as well as polyols, such as glycols, any of which can be utilized individually or in combination. Thus, when R is C₁ to C₁₂ linear or branched alkyl or cycloalkyl, these additives include one or more of the lower alcohols, such as methanol, ethanol, or cyclohexanol, as well as those with more complexity, such as 1-octanol, 2-methyldecan-1-ol, 2-ethylhexan-1-ol, cyclopentanol, cyclohexanol and the like, which can be used in combination. Preferably, such alkyl groups are C₁ to C₆ linear or branched alkyl, i.e., lower alkyl. One requirement of the polar protic foam-enhancing additive is that it is volatile under the conditions used to produce foam. The term "volatile" as used herein means the vapor pressure of the additive is high enough to affect the foam density under the foaming conditions employed.

In the aforesaid additives, the preferred alkyl R groups may be substituted with halo, aryl, alkoxy or hydroxy. Illustrative of the halo-substituted polar protic foam-enhancing additives are 2-chloroethanol, 3-chloropropanol, 4-chlorobutanol and the like. Alkoxy as well as aryl radicals may also be substituted on the alkyl R groups and include phenylmethyl(benzyl), 2-phenylethyl (dihydrocinnamyl), methoxymethyl, ethoxymethyl and the like. Where the substituent on R is a hydroxy group, the alkyl hydroxides are typically called glycols, and such include ethylene glycol, propylene glycol, 1,6-hexandiol and the like.

A particularly preferred form of water, i.e., R═H, is water vapor. It is observed that dry polyimide precursor powder when exposed to such vapor, e.g. humidity in the air, will adsorb sufficient water to give foam having enhanced cellular toughness.

Typically, the polar protic foam-enhancing additive is added to the polyimide precursor at a ratio between about 200 parts by weight precursor to 1 part by weight ROH and about 2 parts by weight precursor to 1 part by weight ROH. Preferably these ratios are from about 100 to 1 to about 5 to 1, most preferably from about 75 to 1 to about 7 to 1.

The "melt" referred to above comprises a heated slurry of a polyimide precursor, generally a particulate, e.g. spray-dried, polyimide precursor powder, in combination with a polar protic foam-enhancing additive. The polyimide precursor comprises one or more tetracarboxylic components and one or more primary, di-, or polyamine components.

Preferably the tetracarboxylic components are esters, predominantly diesters (half esters) of aromatic tetracarboxylic acids and lower alkanols such as methanol, ethanol, isopropanol, propanol, the butanols, the pentanols, the hexanols, and the like. The primary polyamine components employed are preferably aromatic diamines, heterocyclic diamines, or combinations thereof, optionally with a minor proportion of one or more aliphatic diamines.

The organic tetracarboxylic acids or derivatives thereof are preferably based on aromatic tetracarboxylic acids having the general formula: ##STR1## wherein A is a tetravalent organic group, preferably an aromatic group. The tetravalent organic group A is most preferably an aromatic group having a structure such as the following, it being understood the structures shown are illustrative, but not limitative: ##STR2## wherein X is one or more the following: ##STR3## although other aromatic groups are suitable. The tetracarboxylic acid derivatives which may be employed include anhydrides, acid halides, esters, and the like. Of these, esters are preferred and are most generally used for foam production.

Preferred among the tetracarboxylic acid esters are the alkyl esters of 3,3',4,4'-benzophenone tetracarboxylic acid, most preferably the lower (C₁ to C₆ linear or branched chain) alkyl, e.g., methyl or ethyl, diesters thereof. Mixtures of two or more aromatic esters, most preferably predominating in diesters, may be employed, if desired.

It is also possible, in accordance with this invention, that the tetracarboxylic component employed in the manufacture of the polyimide foams be a caprolactam derivative as disclosed in U.S. Pat. Nos. 4,161,477, 4,183,838 and 4,183,839, the contents of which are incorporated herein by reference.

The tetracarboxylic component may also be an N-substituted imido acid ester of the tetracarboxylic acid as disclosed in U.S. Pat. Nos. 4,647,597 and 4,656,198, the contents of which are incorporated herein by reference.

The organic diamine component of the polyimide precursor may be represented by the formula:

    H.sub.2 N--R'--NH.sub.2

wherein R' is an aromatic group containing about 5 to 16 carbon atoms and which may contain at least one hetero atom in the aromatic ring, the hetero atom being nitrogen, oxygen or sulfur. Also included are aromatic groups such as the following, it being understood the structures shown are illustrative, but not limitative: ##STR4##

Representatives of such diamines include:

2,6-diaminopyridine;

3,5-diaminopyridine;

3,3'-diaminodiphenylsulfone;

4,4'-diaminodiphenylsulfone;

4,4'-diaminodiphenylsulfide;

3,3'-diaminodiphenylether;

4,4'-diaminodiphenylether;

meta-phenylenediamine;

para-phenylenediamine;

4,4'-methylene dianiline;

2,6-diamino toluene;

2,4-diamino toluene;

and the like.

It is also possible and sometimes desirable in the preparation of the polyimide precursors, to include in the reaction mixture one or more aliphatic diamines. Such aliphatic diamines are preferably alpha-omega diaminoalkanes having the formula:

    H.sub.2 N--(CH.sub.2).sub.n --NH.sub.2                     (I)

wherein n is an integer from 2 to 16. Representatives of such diamines include 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, etc.

In place of or in addition to the foregoing aliphatic amines, use can be made of aliphatic etherified polyamines such as polyoxypropylene amines having the formula:

    H.sub.2 N--CH(CH.sub.3)CH.sub.2 --[OCH.sub.2 CH(CH.sub.3)].sub.x --NH.sub.2 (II)

wherein x varies from about 1 to about 5.

Other useful primary diamines which may be included in the polyimide precursor include amino-terminated butadienenitrile copolymers having the general formula: ##STR5## wherein R is either a phenylene group or an alkylene group, R₁ is hydrogen or methyl, and x and y are each independently integers ranging from 1 to 25 and n is an integer, preferably below 20. In these copolymers it is preferred that butadiene constitute at least 50% by weight of the butadiene and nitrile monomers. The nitrile monomer copolymerized with the butadiene can either be acrylonitrile or methacrylonitrile. Such copolymers generally have low molecular weights, preferably less than 3,000 to insure that they are sufficiently fluid to react in the formation of the polyimide as well as sufficiently fluid so as to be capable of foaming.

Still another type of primary diamine component which may be included in the polyimide precursor are aromatic amino-terminated silicones, such as those having the general formula: ##STR6## wherein R is a C₂ to C₆ alkylene group, R₁ and R₂ are each independently lower alkyl containing 1 to 3 carbon atoms and n is an integer from 1 to 4.

Another preferred category of diamines which may be utilized in the polyimide precursor are the diesters produced from amino-substituted aromatic carboxylic acids and polymethylene glycols. Such diesters may be represented by the general formula:

    H.sub.2 N--ArCOO--R--OOCAr--NH.sub.2

wherein R is an alkylene group (which may be branched or straight chain) and which preferably contains from 3 to 8 carbon atoms, most preferably trimethylene; and Ar is an aromatic group which may be composed of one or more fused or non-fused benzene rings which in turn may carry suitable substituents (e.g., nitro, alkoxy, etc.) in addition to the primary amino groups.

A few exemplary diesters of this type include:

ethylene glycol-4-aminobenzoic acid diester;

ethylene glycol-3-aminobenzoic acid diester;

ethylene glycol-2-aminobenzoic acid diester;

trimethylene glycol-3-aminobenzoic acid diester;

trimethylene glycol-2-aminobenzoic acid diester;

trimethylene glycol-3-amino-2-nitrobenzoic acid diester;

tetramethylene glycol-3-amino-4-nitrobenzoic acid diester;

tetramethylene glycol-3-amino-5-nitrobenzoic acid diester

tetramethylene glycol-4-amino-2-nitrobenzoic acid diester;

1,5-pentanediol-4-amino-3-nitrobenzoic acid diester;

1,6-hexanediol-5-amino-2-nitrobenzoic acid diester;

neopentyl glycol-4-amino-2-methylbenzoic acid diester;

1,8-octanediol-4-amino-2-propylbenzoic acid diester;

1,9-nonanediol-3-amino-4-methylbenzoic acid diester;

1,10-decanediol-4-(4-aminophenyl)benzoic acid diester;

and the like. Mixtures of such diesters may be employed.

A particularly preferred diester of this type is the diester of trimethylene glycol (1,3-propanediol) and 4-aminobenzoic acid.

The relative proportions of the reactants used in the preparation of the -polyimide precursor can be varied. In general, it is preferred to employ essentially stoichiometric proportions of the tetracarboxylic component and the primary diamine component.

When using a lower alkyl ester of the tetracarboxylic acid, either or both the alcohol produced and the water released during the amidization/imidization reactions, can, in addition to the polar protic foam-enhancing additive, function as the blowing agent during polymerization to form the desired polyimide foams. In addition, use can be made of any of a variety of well-known organic or inorganic blowing agents which can be introduced separately. Variations in the concentration of the blowing agent can be used to achieve specific densities and ILD (indention load deflection) values. However, the use of ROH, as defined above, is preferred as the sole blowing agent, since the foams produced thereby are superior.

The polyimide precursor may also contain various filler and/or reinforcing materials. For example, graphite, glass and other synthetic fibers can be added to the precursor composition to produce a fiber-reinforced product. Glass or phenolic microballoons may be added for density adjustment, if desired, but the microballoons increase density at the expense of flexibility.

It is frequently desirable to also add one or more surfactants to the polyimide precursor, thereby increasing cell stability and uniformity, increasing fatigue resistance, and also making the foam more flexible and resilient. The nature of such surfactants for this use is well known and reported in the patent literature.

When producing foams from tetracarboxylic components other than lower alkyl esters, a blowing agent and/or microballoons generally have been employed in order to achieve a suitable cellular structure.

Although not necessary, for some applications it is desirable that the polyimide precursor contain a suitable quantity of a flame retardant material in order to still further increase the flame resistance of the foam.

Polyimide foams of good quality are produced from the melt obtained by combining a polyimide precursor with the foam enhancing additive, producing a slurry, and then heating the slurry at not more than about 105° C. Thereafter, the melt is heated at one or more temperatures sufficient to obtain a consolidated but friable cellular foam structure, and in at least one other stage this cellular foam structure is heated at one or more higher temperatures sufficient to cure the cellular material into a resilient polyimide foam. Preferably, these stages are conducted in a continuous manner as by supporting the material being foamed on a moving belt or rotating platform associated with appropriate heating apparatus maintained at suitable temperatures (e.g., one or more tunnel heaters, etc., with appropriate temperature zones along the path of travel). Alternatively, a single furnace, oven or other thermal apparatus is employed whereby the material being foamed is kept more or less in one place, and the heat applied thereto is suitably increased during the period of time the melt, developing cellular structure and developed cellular structure are maintained in the apparatus. Any type of apparatus may be used for applying the thermal energy to the melt, to the cellular structure as it is developing therefrom, and to the resultant developed cellular structure. Such equipment includes radiant heaters; furnaces operated on natural gas, LPG, fuel oil, etc.; dielectric heaters; microwave cavities; and the like. However, use of microwave or thermal ovens (e.g., a single circulating air electric oven operated such that its temperature is appropriately increased during the residency of the material being processed therein, or two or more electric resistance heaters positioned in and along the length of a tunnel or cavity with their temperatures suitably set or regulated such that the material being conveyed therethrough encounters increased temperatures during its travel) are preferred because of the lower capital and operating costs involved when using such apparatus. The cured polyimide foam may of course be subjected to a final postcuring at still higher temperatures if desired.

In most cases the foam structure is produced from the melt at temperatures within the range of about 120° to about 180° C., and preferably in the range of about 135° to about 170° C. (about 275° to about 325° F.), and curing is effected at temperatures of at least about 220° C. (preferably at least about 230° C.). However, departures from these ranges are permissible where the circumstances warrant or justify such departures. Usually temperatures above about 425° C. are not used, as thermal degradation of the foam may be encountered, depending of course on the composition of the foam being processed, some foams having greater thermal stability than others.

Any of a number of procedures may be used for forming the melt composition. For example, the tetracarboxylic and diamine components may be mixed with a suitable solvent or liquid diluent and the polar protic foam-enhancing agent in appropriate proportions to form the slurry directly. Similarly, a more dilute solution of the components may be concentrated to the desired solids content as by use of vacuum distillation at a suitably low temperature such that excessive reaction between the tetracarboxylic and diamine components does not occur. Generally speaking, melt compositions with a solids content in the range or about 60 to about 90 weight percent, i.e., a total volatiles content of about 40 to about 10 wt %, are preferred, and those with a solids content in the range of about 65 to about 85 weight percent are particularly preferred.

Having described the basic concepts of this invention, reference is now made to the following examples which serve to still further illustrate the practice and advantages of this invention.

EXAMPLE 1

To a 1.0 liter three-neck, round-bottom glass flask, equipped with a mechanical stirrer, thermometer, heating jacket, and condenser, is added 320 mL (8.0 moles) methyl alcohol, and 24 mL (1.33 moles) distilled water. The solution is thoroughly mixed and 322.23 g (1.0 mole) benzophenonetetracarboxylic acid dianhydride (BTDA) are added with stirring. This mixture is then heated to reflux to form the methyl ester of the BTDA. Once the esterification reaction is complete, the clear solution is cooled to below 40° C. and 158.6 g (0.8 mole) of methylene dianaline (MDA) are added. After the MDA has completely dissolved, 21.8 g (0.2 mole) of diaminopyridine (DAP) are added and mixed until dissolved. This is followed by the addition of 7.0 grams (1.5 percent by weight of the polyimide solids) of a silicone glycol surfactant, Dow Corning 193. The solution is thoroughly mixed to yield a liquid polyimide precursor.

The liquid polyimide precursor is processed into powder, i.e., a particulate material, using a spray dryer or a vacuum dryer.

EXAMPLE 2

About 300 g of polyimide precursor (prepared as described in Example 1) was placed in a "sigma-blade" mixer, along with an appropriate amount of water and heated to about 55° C. The mixer was then turned on and the materials mixed (about 1 hour) until a homogeneous paste, resembling taffy, was obtained. The melt was then poured out onto a heat resistant, microwave compatible (e.g. teflon coated glass cloth) substrate. The melt was then placed in a Gerling-Moore laboratory microwave oven operating at a frequency of 2450 mHz for 20 minutes at 200 watts for preheating. Once the melt was thoroughly preheated the melt was foamed for 20 minutes at 220 watts followed by curing for 20 minutes at 2200 watts followed by 60 minute thermal post-cure at 500° F. A low density, resilient polyimide foam is obtained.

The effect of varying the amount of water added is described in Table 1.

                  TABLE 1                                                          ______________________________________                                         Conc Water                                                                              % Water by Karl       Tensile.sup.b                                                                         Thermal.sup.c                            Added (%)                                                                               Fisher Analysis                                                                             Density.sup.a                                                                           Strength                                                                              K                                        ______________________________________                                         0 (Powder                                                                               0.8          0.68     12.3   0.28                                     only)                                                                          3        3.1          0.51     11.1   0.30                                     5        4.1          0.46     6.5    0.34                                     ______________________________________                                          .sup.a lb/ft.sup.3                                                             .sup.b psi                                                                     .sup.c (BTU · in)/hr · ft.sup.2 · °F.  

EXAMPLE 3

A melt composition, having a solids content of 65-70 percent and a surfactant concentration of 4.0 percent, was prepared using the method described in Example 2 and was mixed with varying amounts of carbon fibers, ranging in concentration from 0.5-50 percent. These filled resins were then foamed and cured as described in Example 2 to obtain a series of reinforced foams. It was found that as the concentration of fiber increased the density of the foam increased, and the foam became more rigid.

EXAMPLE 4

In a 12 1 three neck flask, equipped with a stirrer, 4.189 kg of 3,3',4,4'-benzophenone tetracarboxylic acid (BTDA), 1.440 kg of CH30H and 90 g of water were heated to reflux. Once the solution became clear heating was continued for an additional hour to insure complete conversion to the diester derivative. The diester solution was transferred to a 1 gallon capacity "sigma-blade" mixer, and 1.838 kg of 4,4'-methylene dianiline (MDA) was added and mixed for 15 minutes. Then 434 g of 2,6-diaminopyridine (DAP) was added and the heating to jacket set at 60° C. Heating and mixing continued for 26 minutes, temperature at 60° C., and 90.1 g of surfactant was added along with 296 g H₂ O. The resultant melt was mixed for an additional hour.

The melt was then poured out, and four buns (about 1500 g each) foamed in a Gerling-Moore laboratory microwave oven. The melt was preheated 3 minutes at 1400 watts, followed by foaming for 30 minutes at 2200 watts, followed by curing for 15 minutes at 4000 watts and a thermal oven post-cure of 60 minutes at 500° F.

The articles were low density, flexible, resilient polyimide foams.

EXAMPLE 5

An analysis of the powder particulate polyimide precursor prepared in an identical manner as that of Example 1 revealed that it had a total volatiles content of 20.6%, with 0.58% of such Volatiles being water. "Total volatiles", wherever used in this application, is the percent weight lost from a sample heated for 30 minutes at a temperature of 260° C. It is believed to include both the free volatiles, such as solvent, as well as the bound volatiles which arise from amidization and imidization reactions; "free volatiles", wherever used in this application, is the percent weight lost from a sample heated at 50° C. under vacuum (30 in. Hg) for 3 hr. The powder was exposed to the air (ambient temperature and pressure) for 30 minutes. Analysis of the resulting powder gave total volatiles of 20.9%, with 2.74% being water. A total water gain of 2.16 wt % therefore occurred, with total volatiles slightly elevated i.e., a loss in the volatile components of 1.86 wt % occurred.

The resulting polyimide precursor powder isolated from the above procedure was foamed by the same process as shown in Example 4. The polyimide foam bun produced had a tensile strength of 10.33 psi (average of two); a density of 0.589 lbs/ft³ (average of two); and a thermal K of 0.305 (BTU·in)/(hr·ft² ·°F.)

EXAMPLE 6

To 35 lb. quantities of polyimide precursor powder prepared as described in Example 1 was added 700 ml of a mixture of various ratios of water and methanol in a mixer as described in Example 2. The resultant melts were then foamed as set forth in the latter Example. The results obtained are set forth in Table 2.

                  TABLE 2                                                          ______________________________________                                         MEOH           H.sub.2 O                                                                             Foam density                                             (ml)           (ml)   (lb/ft.sup.3)                                            ______________________________________                                          0             700    0.38                                                     100            600    0.43                                                     200            500    0.44                                                     ______________________________________                                    

EXAMPLE 7

A polyimide precursor is prepared in the manner of Example 1, except that the molar ratio (MDA)/(DAP) is 0.715/0.305 rather than 0.8/0.2, and the free volatiles content of the powder particulate precursor is less than about 5 weight percent. Varying amounts of water are added to 500 g portions of the resultant powder, producing a series of slurries. The slurries are mixed and heated at 50° C. to 62° C. for 20 minutes, producing a series of melts. The melts are then foamed and cured by heating the melts first for 30 mins. in a circulating air, convection oven at 68° C., followed by heating in the Gerling-Moore laboratory microwave oven of Example 2 for 20 min. at 1400 watts, followed by 40 min. at 2800 watts. Lastly, the resultant foams are heated in the convection oven for 30 min. at 260° C. The result is a series of foams with a range of densities, as shown in Table 3 and FIG. 1.

                  TABLE 3                                                          ______________________________________                                         Slurry       Cured Foam                                                        wt %    % total  density tensile                                               Added H.sub.2 O                                                                        volatiles.sup.a                                                                         lb/ft.sup.3                                                                            psi   % LOI.sup.b                                                                           thermal K.sup.c                          ______________________________________                                          0      19.97    0.54    7.1   37     0.312                                     3      20.68    0.48    9.9   34     0.290                                     5      21.23    0.38    6.6   36     0.322                                     7      21.93    0.28    6.4   36     0.363                                     9      22.13    0.25    2.1   32     0.373                                    11      23.60    0.25    2.4   33     0.400                                    11      25.46    0.21    2.6   32     0.383                                    13      26.92    0.21    2.0   32     0.408                                    15      28.09    0.17          30     0.457                                    17      30.54    0.15    2.0   31     0.424                                    ______________________________________                                          .sup.a Percent weight lost from sample heated for 30 min. at 260°       C.                                                                             .sup.b Limiting Oxygen Index                                                   .sup.c (BTU · in)/(hr · ft.sup.2 · °F.)

EXAMPLE 8

Using the methods and polyimide precursor of Example 7, except that varying amounts of several alcohols, equimolar in amount to 3.0 wt % water, are added to the powder, producing a series of slurries. The slurries are mixed and heated at 60° C. to 63° C. for 20 minutes, producing a series of melts which are foamed and cured in the manner of Example 7. The results are shown in Table 4.

                  TABLE 4                                                          ______________________________________                                         Slurry                                                                         wt %                Cured Foam                                                        Added   % total  density                                                                              tensile      ther-                               ROH    ROH     volatiles.sup.a                                                                         lb/ft.sup.3                                                                          psi   % LOI.sup.b                                                                           mal K.sup.c                         ______________________________________                                         HOH    3.0     20.68    0.48  9.9   34     0.290                               MeOH   5.33    27.20    0.38  5.3   33     0.342                               EtOH   7.67    24.62    0.36  5.0   34     0.339                               i-PrOH 9.99    25.54    0.36  7.7   34     0.323                               i-BuOH 12.38   26.62    0.32  5.6   33.5   0.372                               ______________________________________                                          .sup.a Percent weight lost from sample heated for 30 min. at 260°       C.                                                                             .sup.b Limiting Oxygen Index                                                   .sup.c (BTU · in)/(hr · ft.sup.2 · °F.)

As can be seen from the foregoing, the melt compositions used in the practice of this invention comprise at least (i) one or more organic tetracarboxylic compounds, (ii) one or more organic diamine compounds co-reactive therewith, and (iii) a polar, protic foam-enhancing additive. Preferably the diamines include at least one aromatic and/or aromatic heterocyclic primary diamine. Components (i) and (ii) are usually present in the mixture in essentially stoichiometric (substantially equal molar) quantities. Most preferably, such melt compositions further include a suitable quantity of a surfactant, most preferably a silicone glycol surfactant.

It will be apparent that this invention is susceptible to considerable variation in its practice without departing from the spirit and scope of the appended claims, the embodiments described hereinabove being merely exemplary of its practice. 

We claim:
 1. A method for producing a polyimide foam bun of a desired density within the range from about 0.1 lb/ft³ to about 0.7 lb/ft³ and which is substantially free of solid blowing agent or residue thereof which comprises forming a polyimide precursor by combining a lower alkyl ester of a tetracarboxylic acid with at least one aromatic and/or heterocyclic aromatic primary diamine; adding to said polyimide precursor at least one polar protic foam-enhancing additive of the formula ROH, wherein R is selected from the group consisting of hydrogen, and C₁ to C₁₂ linear or branched alkyl or cycloalkyl, optionally substituted with halo, aryl, alkoxy or hydroxy, producing a slurry comprising between about 17 weight percent and 0 weight percent, respectively, of added ROH, with the proviso that said foam-enhancing additive is volatile at the foaming temperature to be employed; heating the slurry at a temperature to produce a melt as a homogeneous transparent solution or opaque suspension; and then foaming and curing said melt.
 2. The method of claim 1 wherein said slurry is heated at a temperature between about 50° C. and 105° C., producing said melt.
 3. The method of claim 1 wherein said polyimide precursor is a particulate solid.
 4. The method of claim 1 wherein ROH is water.
 5. The method of claim 4 wherein the water is added by exposing the particulate polyimide precursor to water vapor.
 6. The method of claim 4 wherein the polyimide precursor comprises the lower alkyl diester of at least one aromatic tetracarboxylic acid, and at least one aromatic or heterocyclic primary diamine.
 7. The method of claim 6 wherein the diester is a lower alkyl ester of benzophenone tetracarboxylic acid.
 8. The method of claim 6 wherein the diamine comprises 4,4'-methylenedianiline.
 9. The method of claim 1 wherein the precursor includes a surfactant.
 10. The method of claim 6 wherein the diester is a lower alkyl ester of benzophenone tetracarboxylic acid and the diamine is primarily or entirely 4,4'-methylenedianiline.
 11. The method of claim 6 wherein the particulate polyimide precursor has a free volatiles content of less than about 5 weight percent.
 12. The method of claim 1 wherein the polyimide precursor includes a silicone glycol surfactant.
 13. The method of claim 6 wherein the diester is the dimethyl ester of benzophenone tetracarboxylic acid or the diethyl ester of benzophenone tetracarboxylic acid, the diamine is predominately a mixture of 4,4'-methylenedianiline and a diaminopyridine, and wherein the precursor includes a silicone glycol surfactant.
 14. The method of claim 14 wherein the particulate polyimide precursor has a free volatiles content of less than about 5 weight percent.
 15. The method of claim 14 wherein said melt is prepared by heating a slurry comprising particulate polyimide precursor and said polar protic foam-enhancing additive at a temperature between about 50° C. and about 105° C.
 16. The method of claim 14 wherein R is selected from hydrogen and lower alkyl. 